Vibration damped sandwich systems

ABSTRACT

A VIBRATION DAMPED SANDWICH SYSTEM COMPRISING TWO HARD PLATES AND INTERPOSED BETWEEN THE PLATES A VIBRATION DAMPING INTERLAYER COMPRISING GRAFT POLYMERS OF STYREN OR OPTIONALLY STYRENE WITH SMALL AMOUNTS OF A COPOLYM ERIZABLE CARBOXYLIC ACID, ESPECIALLY ACRYLIC AND/OR METH ACRYLIC ACID, ON A COPOLYMER OF 30 TO 40% BY WEIGHT OF VINYL ACETATE, 30-40% BY WEIGHT OF N-BUTYL ACRYLATE, 30 TO 10% BY WEIGHT OF DIBUTYL MALEATE AND ABOUT 10% BY WEIGHT OF CROTONIC ACID.   D R A W I N G

United States Patent O U.S. Cl. 161-166 4 Claims ABSTRACT OF THE DISCLOSURE A vibration damped sandwich system comprising two hard plates and interposed between the plates a vibration damping interlayer comprising graft polymers of styrene or optionally styrene with small amounts of a copolymerizable carboxylic acid, especially acrylic and/or methacrylic acid, on a copolymer of 30 to 40% by weight of vinyl acetate, 30-40% by weight of n-butyl acrylate, 30 to by weight of dibutyl maleate and about 10% by weight of crotonic acid.

The present invention provides vibration damped sandwich systems having interlayers made of graft polymers of styrene, optionally of styrene with a copolymerizable carboxylic acid (acrylic and/or methacrylic acid), on vinyl acetate/n-butyl acrylate/dibutyl maleate/crotonic acid copolymers.

It is known from South African specification No. 5,269, that highly valuable vibration damping materials of a broad temperature band suitable for damping bending vibrations of metal sheet constructions can be prepared by copolymerizing monomers whose homopolymers differ in their second order transition temperature by at least C. The above specification also reports that as vibration damping materials having a broad temperature band there can be used, among other, predominantly amorphous copolymers of esters of alcohols with 4 to 12 carbon atoms with acrylic and maleic acid and vinyl esters of fatty acids with 2 to 3 carbon atoms, for example vinyl acetate/n-butyl acrylate and/or dibutyl maleate copolymers.

It has now been found that graft polymers of styrene or optionally styrene with small amounts of a copolymerizable carboxylic acid, especially acrylic and/or methacrylic acid, on copolymers of vinyl acetate, suitable esters of unsaturated polymerizable carboxylic acids and an unsaturated copolymerizable acid, preferably crotonic acid, have outstanding damping properties, above all with regard to the width of the temperature band, and are especially suitable for the vibration damping of sandwich systems of hard plates, in particular metal sheets. Suitable esters are especially those of acrylic acid and maleic acid with alcohol components having 3 to l2 carbon atoms, preferably n-butyl acrylate and dibutyl maleate. With these polymers, the vibration damping effect of which critically depends on the weight proportion of the monomers, very broad damping curves can be obtained having very high maximum damping values. From the economical point of view they have the advantage of being rather cheap.

The graft polymers of styrene or optionally styrene with a copolymerizable carboxylic acid (acrylic and/or methacrylic acid) on copolymers of 30 to 40% by weight of vinyl acetate, 30 to 40% by weight of n-butylacrylate, 30 to 10% by weight of dibutyl maleate and about 10% by weight of crotonic acid are prepared by free radical initiated polymerization at a temperature in the range of from about 60 to about 180 C. by dissolving the copolymer in monomeric styrene, optionally a mixture of styrene with a copolymerizable carboxylic acid (acrylic and/ or methacrylic acid), containing a catalyst. Especially good results are obtained with graft polymers of 40 to by weight of styrene or styrene with a copolymerizable carboxylic acid (acrylic and/ or methacrylic acid) on 60 to 20% by weight of the copolymers specified above, for example a graft polymer of 50% by weight of styrene or 50% by weight of a mixture of 90% by weight of styrene and 10% by weight of acrylic acid on 50% by weight of the aforesaid copolymer. As polymerization initiator tertiary butylhydroperoxide may be used.

The present invention provides sandwich systems of hard plates, in particular metal sheets, having vibration damping, self-adherent interlayers of graft polymers of styrene, optionally of styrene with a copolymerizable carboxylic acid (acrylic and/or methacrylic acid), on vinyl acetate/n-butyl acrylate/dibutyl maleate/crotonic acid copolymers, for which interlayers there are used graft polymers of 40 to 80% by weight of styrene or styrene together with a copolymerizable carboxylic acid (acrylic and/or methacrylic acid) on 60 to 20% by weight of copolymers consisting of 30 to 40% by weight of vinyl acetate, 30 to 40% by weight of n-butyl acrylate, 30 to 10% by weight of dibutyl maleate and 5 to 10% by weight of crotonic acid.

FIGS. 1a and 1b of the accompanying drawings are plots against temperature of the loss factor dcomb to illustrate the superior efficiency of the novel systems. The curve in FIG. 1a shows the loss factor dcomb of a metal sheet arrangement of the invention as a function of temperature. For comparison, one of the most effective vibration damping materials known for metal sheet arrangements was used, namely a copolymer of 63% by weight of vinyl acetate and 37% by weight of dibutyl maleate containing as plasticizer 15% by weight of 2-ethylhexyl phthalate and 15 by weight of tricresyl phosphate, calculated on the mixture (curve FIG. lb). The copolymer of curve lb was a thermoplastic adhesive especially suitable for producing vibration damped metal sheet sandwich systems comprising two outer metal sheets and a self-adherent thermoplast as damping interlayer. Systems of this type provide a damping eect which is extremely high in its maximum and cannot be exceeded for physical reasons (cf. H. Oberst and A. Schommer, Kunststoffe, volume 55, page 634 (1965), especially FIG. 9). In a symmetrical arrangement comprising two metal sheets, each having a thickness of 0.5 millimeter, and an interlayer 0.3 millimeter thick, the loss factor dcomb of the combined system, measured in the bending wave method (cf. for example H. Oberst, L. Bohn and F. Linhardt, Kunststoffe, volume 51, page 495 (1961) almost reaches the value dwmb of l. The known metal sheet damping by one-side damping coatings which are applied by spraying, trowelling or bonding in the form of layers of so-called vibration damping materials show loss factors generally of less than dcomb=0.2 with technically reasonable thicknesses or ratios of coating mass to plate mass of the combined system. With metal sheet sandwich systems which gain growing importance in recent times, it is possible to obtain damping values that are increased by a multiple, as shown by the present example, when the interlaycr material has the appropriate composition and thickness.

The temperature band width of the damping of the metal sheet sandwich system does not only depend on the viscoelastic properties of the interlayer and the steel sheets but also to a considerable extent on the geometry of the arrangement, i.e. on the ratio of the layer thicknesses (cf. loc. cit (1965), FIGS. 8 to 10). With metal sheet sandwich systems the band width is advantageously defined as the range of the temperature interval within which the value dcomb=-05 is exceeded. The damping of metal sheets which are not damped by additional vibration damping means in metal sheet construction of various types corresponds to values dcombOl. The reference value dcomb of 0.05 thus means a considerable increase in the damping effect (by about 15 db (decibel)) as compared to the nil damping dcomb=0-01 In the curve shown in FIG. 1b the reference value dcomb of 0.05 is exceeded in the mainly interesting frequency range of from 100 to 1,000 c.p.s. (Hz.) at temperatures ranging from about 0 to 50 C. The temperature band width thus corresponds to about 50 C. Sandwich systems of this type are suitable for many technical fields of application. By modifying the content of plasticizer, it is possible to shift the temperature band of a high damping effect to higher temperatures and thus to adapt the material to special technical uses, for example in machine units operating at elevated temperatures. This example of a metal sheet sandwich system comprising a self-adherent interlayer having optimum properties 0f a vibration damping material with a broad temperature band prepared by copolymerizing appropriate monomeric compounds has hitherto not been surpassed by other arrangements of similar kind and may be taken as standard for judging the acoustic efficiency of the system according to the invention.

FIGS. 1a and 1b show the temperature curves of the loss factor dcomb of metal sheet sandwich systems comprising steel sheets of a thickness of 0.5 millimeter each and damping interlayers of a thickness of about 0.7 and 0.3 millimeter (in FIG. lb) for a frequency of 100 c.p.s. and 1,000 c.p.s.

The curves were measured with sandwich systems the interlayers of which consisted of (1a) A graft polymer of 50% by weight of styrene on 50% by weight of a copolymer of 35% by weight of vinyl acetate, 35% by weight of n-butyl acrylate, 20% by weight of dibutyl maleate and by weight of crotonic acid (according to the invention).

(1b) A copolymer of 63% by Weight of vinyl acetate and 37% of dibutyl maleate containing as plasticizer by weight of 2-ethylhexyl phthalate and 15% by weight of tricresyl phosphate, calculated on the mixture.

The arrangement (la), whose monomer proportion lies in the optimum range has a very broad temperature band width with relatively high maximum damping values that come near to the values of arrangement (1b). With the arrangement (la) the centre of damping is at 50 to 65 C. with maximum damping values of about 0.5 to 0.7. The temperature band width is about 140 C. for 100 c.p.s. and about 105 for 1,000 c.p.s. In arrangement (la), the slow decrease of the damping towards high and low temperatures is especially favourable. The reference value dcomb of 0.05 is exceeded at a temperature in the range of from about 5 to about 130 C. As compared with the standard system (lb), arrangement (la) has an appreciably broader temperature band width with excellent vibration damping properties at temperatures above 40 C. so that it is especially interesting for quite a number of applications, for example in machines and appliances operated at normal and elevated temperatures. Owing to its content of 10% by weight of crotonic acid the graft polymer used in arrangement (1a) can be crosslinked by reaction with a bifunctional or trifunctional compound (for example a compound containing a plurality of epoxide, isocyanate or similar groups), whereby the softening range and therewith the range of high damping can be noticeably shifted towards higher temperatures.

A special advantage of the vibration damping material of the present invention resides in the fact that it may be applied continuously during the mass production of the metal sheet sandwich systems. For this purpose the damping material can be applied in the form of 1) the finished graft polymer or in the form of (2) a solution of the specified copolymer in the styrene or mixture of styrene with acrylic and/or methacrylic acid containing the polymerization initiator and the graft polymer can be produced by a thermal treatment at a temperature of from about 60 to about 180 C. of the sandwich system comprising the solution as interlayer.

It is a. thermoplastic adhesive which may be applied to the metal sheets by trowelling, brushing or pouring at elevated temperature. The sandwich system may then be advantageously cooled under pressure between rollers. Except for degreasing the metal sheets do not require a preliminary treatment and further adhesive. Owing to the content of the graft polymer of crotonic acid, acrylic acid and/or methacrylic acid degreasing may even be dispensed with. The adhesion is very good.

The vibration damping material of the present invention has a good resistance to flow. The metal sheet sandwich system may, within broad limits, be processed as normal metal sheets, that is they may be creased, bent, shaped, welded and riveted. In this manner laminated systems are obtained having a damping weight and temperature range of damping which makes them well suitable for many applications at normal and elevated temperatures.

Minor amounts of fillers, for example for improving the electric conductivity (improvement of resistance welding) may be incorporated in the vibration damping materials. In order not to affect the damping effect adversely it is advantageous to use less than 1% by weight, preferably less than 0.5% by weight of filler, calculated on the polymer. Suitable fillers are, for example, heavy spar, slicic acid, graphite and soot.

The metal sheet sandwich system suitably has a total thickness in the range of from 1 to 6 millimeters. The interlayers may have a thickness of 0.1 to 1 millimeter, preferably 0.2 to 0.5 millimeter. A maximum damping effect is obtained with symmetrical laminated systems. With an equal weight, asymmetrical laminated systems have, however, a higher stiffness in flexure and strength. Asymmetrical laminated systems are, therefore, preferred for those applications which require a high strength with respect to the weight. The ratio of the thicknesses of the outer plates or metal sheets is preferably within the range of from 1:1 to 1:4.

FIG. 2 of the accompanying drawings shows sandwich systems with symmetrical arrangement (a) and asymmetrical arrangement (b) in which the interlayer 2 is interposed between the two outer plates or metal sheets I.

What is claimed is:

1. A vibration damped sandwich system comprising two hard plates and interposed between the plates a vibration damping self-adherent interlayer comprising a graft polymer of 40 to 80% by weight of styrene or a mixture of styrene with 0.1 to 10% by weight of acrylic and/or methacrylic acid, calculated on the styrene, on 60 to 20% by weight of a copolymer of 30 to 40% by weight of vinyl acetate, 30-40% by weight of n-butyl acrylate, 30 to 10% by weight of dibutyl maleate aud 5 t0 15% by weight of crotonic acid.

6 2. A vibration damped sandwich system as claimed in FOREIGN PATENTS claim 1, wherein the hard plates are metal sheets. 890 249 2/1962 Great Britain 3. A vibration damped sandwich system as claimed in 499277 3/1950 Belgium claim 1, wherein the graft polymer of the interlayer con- 1,455,141 8/1966 Franca tains up to 1% by weight of a filler, calculated on the 5 1200 458 11/1965 Germany graft polymer.

4. A vibration damped sandwich system as claimed in OTHER REFERENCES claim 1, wherein the ratio of the thicknesses of the Chemical Abstracts, v01, 55, 403519. plates lies preferably in the range of from 1:1 to 1:4.

10 MORRIS LIEBMAN, Primary Examiner References Cited S. L. FOX, Assistant Examiner UNITED STATES PATENTS 3,271,188 9/1966 Aibert et a1 181-3101 U-S-Cl-X-R- 3,399,103 s/196s saiyer er a1. 161 165 16.1-165,21s;26o so.s1,885 

